1. Field of the Invention
The invention relates to improved sensing coils for fiber optic rotation sensing devices. More particularly, this invention pertains to an improved fiber optic sensing coil and tooling therefor.
2. Description of the Prior Art
Fiber optic rotation sensing devices, such as gyroscopes, comprise two main components, (1) an interferometer (including a light source, beamsplitter, and detector) and (2) a fiber optic sensing coil, that are mounted to a rotatable platform. Light from the interferometer's light source is split by the beamsplitter into two beams, each of which is coupled into an opposed end of the sensing coil. The interferometer and associated electronics process the phase relationship between the two interfering, counter-propagating beams of light when they emerge from (opposite ends of) the coil. A phase shift difference between the two beams results from (1) coil rotation and (2) so-called "environmental" factors.
Environmental factors include such variables as temperature, vibration (both acoustical and mechanical), and magnetic fields. These factors can induce phase shifts between the counter-rotating beams which are indistinguishable from those that are induced by rotation. Environment-induced phase shifts reflect the effects of such factors upon variations in the optical light path that each beam encounters as it travels through the coil. Environmental factors are both time varying and unevenly distributed throughout the coil. As it takes a finite amount of time for a beam's wave front to pass a particular point, the effects upon the two beams are unequal, producing an undesirable (phase-shift) effect that is not later cancelled out.
In the past, particular coil winding arrangements have been utilized in attempts to minimize the effects of environmental interference factors and thereby enhance the accuracy of the sensor. Symmetric windings are arranged so that the center of the fiber forms the central turns of the coil while the ends of the fiber form its outer turns.
While the advantages of symmetric windings are recognized, this desirable arrangement has proven difficult to realize. At the present time, fiber optic sensing coils are commonly wound in a helix-type configuration. In a helix winding turns of the fiber cross the coil axis at ninety degrees plus or minus a so-called "helix angle". In the first layer of such a sensing coil, the turns are generally wound from left to right, forming a left-handed helix. The fiber of the second layer, which rests atop the first layer, runs from right to left to form a right-handed helix. Unfortunately, the reversal of the senses of the windings of adjacent layers tends to destroy the desired symmetry and regularity of the coil. The windings of a layer tend to "follow" the reverselydirected "grooves" that are formed between adjacent turns of the underlying layer. As a result, it is quite difficult to maintain the desired reversal of the senses of the windings of adjacent layers. Furthermore, as the turns of a layer alternate between lying atop and following the "grooves" of the preceding layer, the space between turns widens to an extent that turns from overlying layers begin to merge with those of preceding layers. The departure from an ideal, reversely-wound helical configuration increases as the number of overlying layers increases. At a radial distance of twelve to fourteen layers it is virtually impossible to distinguish the senses of the turns or the turns of adjacent layers. As a consequence, the desired cancellation of the effects of environmental disturbances is limited by the very configuration of the conventional, helically-wound fiber optic sensing coil.